A number of articles in widespread use comprise thin films of silver and/or copper. Such films are often used, for example, to form the reflective surfaces of mirrors. Such films are also commonly used to impart partial reflectivity to glass surfaces for use, for example, in windows, decorative structural materials, and the like.
Depending on the intended use, these metallic films may vary widely in thickness. For example, the metallic layer on mirrors typically varies from about 3 to about 10 microinches (about 76.2 m.mu. to about 254 m.mu.), whereas the partially reflective, metallic films on windows where the films serve as solar reflectors typically range from about 4.times.10.sup.-8 inches to about 20.times.10.sup.-8 inches (about 1 m.mu. to about 5.1 m.mu.).
While useful for decorative and reflective purposes, silver and/or copper metallic films are readily subject to discoloration and degradation due to oxidation and heat and attack by a number of air borne contaminants such as chlorides, sulfides and other contaminants.
Although these metallic films vary in thickness, they are always quite thin. In the context of the present invention the metallic films generally have a thickness of up to about 10 microinches (about 254 m.mu.). Presumably due to the thinness of the films, materials known to inhibit corrosion and tarnishing of the bulk metals are not sufficiently effective to prevent tarnishing and corrosion of articles on which these thin metallic films are present. In fact, in some cases such known "inhibitors" for the bulk metals, such as benzotriazole for example, actually increase the rate of deterioration of the thin film.
In most applications, especially where thin silver and/or copper films are used for decorative and partially reflective films on windows, even slight deterioration of the film over extended periods of time poses a significant problem. One attempt to solve this problem has been to coat the metallic film with an organic coating composition. However, while providing some measure of protection, even this expedient is not sufficient to prevent eventual discoloration and deterioration of the metallic film. These organic coating compositions apparently are sufficiently permeable to various contaminants which cause and promote deterioration of the metallic film, that they do not provide adequate long term protection for thin silver and/or copper films. Moreover some organic coating compositions contain reactive groups which react with the metal and discolor the film. Additionally, because of various close tolerances which must be met when thin metallic films are used in window applications, the use of organic coatings is not feasible.
U.S. Pat. No. 3,382,087 to Ostrowski discloses the treatment of articles coated with thin silver and copper films with aminotriazoles, aminotetrazoles, aminoindazoles and indazole itself.